1. Field of the Invention
The present invention relates to a crystalline semiconductor for use in thin film devices such as thin-film insulated-gate field-effect transistors (hereinafter referred to simply as "thin film transistors" or "TFTs"), and to a process for fabricating the same.
2. Prior Art
Thin films of crystalline silicon semiconductor for use in thin film devices such as TFTs known heretofore have been fabricated by crystallizing an amorphous silicon film formed through plasma CVD (chemical vapor deposition) or thermal CVD, using an apparatus such as an electric furnace maintained at a temperature of not lower than 600.degree. C. for a duration of 12 hours or longer. Thin films of crystalline silicon semiconductor having sufficiently high quality (for example, an excellent field effect mobility and a high reliability) are available only after subjecting the amorphous film to a heat treatment for a still longer duration.
However, those prior art processes for obtaining thin films of crystalline silicon semiconductor suffer various problems yet to be solved. One of the problems is the low throughput which increases the process cost. For instance, if a duration of 24 hours is required for the crystallization step, and by considering that the process time for a single substrate is preferably within 2 minutes, 720 substrates must be processed at a time. However, the maximum number of substrates which can be treated at a time in an ordinary tubular furnace is limited to 50; in a practical treatment using only one apparatus (reaction tube), it has been found that a single substrate requires 30 minutes to complete the treatment. In other words, at least 15 reaction tubes are necessary to complete the reaction per single substrate in 2 minutes. This signifies that such a process increases the investment cost and therefore increases the product price due to a too large depreciation for the investment.
The temperature of the heat treatment is another problem to be considered. In general, a TFT is fabricated using a quartz glass substrate comprising pure silicon oxide or an alkali-free borosilicate glass substrate such as the #7059 glass substrate manufactured by Corning Incorporated (hereinafter referred to simply as "Corning #7059 substrate"). The former substrate has such an excellent heat resistance that it can be treated in the same manner as in a conventional wafer process for semiconductor integrated circuits. However, it is expensive, and, moreover, the price increases exponentially with increasing area of the substrate. Thus, at present, the use of quartz glass substrates is limited to TFT integrated circuits having a relatively small area.
On the other hand, alkali-free borosilicate glass substrates are inexpensive as compared to those made of quartz glass, however, they have shortcomings with respect to their heat resistance. Since an alkali-free glass substrate undergoes deformation at a temperature in the range of from 550.degree. to 650.degree. C., and more particularly, since a readily available material undergoes deformation at a temperature as low as 600.degree. C. or even lower, any heat treatment at 600.degree. C. causes an irreversible shrinkage and warping to form on the substrate. These deformations appear particularly distinctly on a substrate having a diagonal length of more than 10 inches. Accordingly, it is believed requisite to perform the heat treatment on a silicon semiconductor film at a temperature of 550.degree. C. or lower and for a duration of within 4 hours to reduce the entire process cost.